Where does the backup camera trigger line get connected at the stereo unit connect to?

The backup camera trigger line which, when it gets hot, triggers the stereo to switch to display the backup camera.  It is usually marked "reverse camera" and the wire color - 

Alpine iLX-W65 Orange/White
Boss Audio BE7ACP Violet/White
Kenwood  All models Purple/White
Pioneer AVIC-HD3 Orange/White
SONY XAV-AX150 Orange/White


Run a short wire from the "backup light positive" fuse box to the  "reverse camera".  Or use the following: 

Connect (1) to the purple wire and the (2) to backup light positive – will trigger the stereo unit to switch to the backup camera when the backup light comes on – (1) is continuous to (2).

Thant means two red wires (2) and (3) will be connected to the backup light positive. One (3) to power the camera and the second one (2) to trigger the stereo.